What are the units for camera shift?
It seems to be in percentage or fraction of the render size (in pixels).

That means that a lens shift of 1 shifts the camera exactly one frame unit in a certain direction, making it frame exactly outside the previous default framed area.

This is measured however in relation to the largest dimension of the rendered frame size. So lets say you are rendering in Full HD, that is 1920 x 1080 pixel image; a frame shift if 1 unit will shift exactly 1920 pixels in any direction, that is up/down/left/right.

This may seem obvious while shifting sideways towards the X axis, but might seem random while shifting in the Y axis.